In PET imaging, a subject is injected with a radiopharmaceutical which targets particular tissues typically through absorption based on a metabolic activity. As the radiopharmaceutical decays, positrons are emitted which annihilate in contact with an electron to form a pair of photons emitted 180° opposite along a line of response (LOR). The emitted gamma photons are recorded by PET detectors surrounding the subject. The locations of the annihilation events are computed from the recorded gamma photons which provide an image of the tissues targeted by the radiopharmaceutical. The emitted photons are affected by the density of tissues between the point of the annihilation event and the detector by either absorption or deflection. The recorded amount of radiopharmaceutical present in tissue is attenuated from the actual amount by the tissue density along the LORs. Correction for attenuation of PET image data seeks to accurately identify the tissue density at each voxel with an attenuation correction map for the subject being imaged.
Developing attenuation correction maps has been performed with X-ray radiation devices such CT scanners where recorded levels of X-ray radiation attenuation or transmission used to reconstruct images correlate strongly with tissue density. Newer techniques seek to use magnetic resonance systems (MR based attenuation correction—“MRAC”) which avoid exposure of the subject to X-ray radiation. However, magnetic resonance imaging does not inherently differentiate tissue density.
Water and fat images are used to segment tissues for tissue classification. Current MR techniques, such as Dixon or mDixon, for concurrently obtaining water and fat images are typically optimized to produce the best possible spatial resolution in a given data acquisition time. In the Dixon technique, more fully described in U.S. Publication No. 2010/0052674, an in-phase image (0°) and an out-of-phase image (180°) are reconstructed from data collected at two echo times in a pulse sequence timed such that the phase difference between water and fat is usually 180° at the first echo time and 0° or (360°) at the second echo time. In the mDixon technique, more fully described in International Patent Application PCT/IB2010/050745, the constraints on the phase difference between water and fat at the first and at the second echo time are relaxed, allowing two arbitrary echo times with phase differences between water and fat other than 0° and 180°.
In the Dixon technique, long TRs (repetition times) may be experienced which results in long data acquisition times. For example, when imaging a station of a whole body image, the time needed to collect all data exceeds a typical subject's capability to hold his or her breath, leading to image artifacts induced by respiratory motion and thus in tissue misclassification in attenuation maps at a voxel level.
Typical PET systems provide a resolution of ≥2 mm, whereas typical MR systems achieve a resolution of ≤1 mm. In consequence, the MR system usually classifies each voxel of the tissue at a higher resolution in the attenuation map, which is then later aggregated, averaged, interpolated, etc. to map to the coarser resolution of the PET system to perform the attenuation correction.